MMR vaccine and idiopathic thrombocytopaenic purpura

Aims To estimate the relationship between idiopathic thrombocytopaenic purpura (ITP) and the measles, mumps and rubella (MMR) vaccination in children; calculating the relative risk estimate for ITP with in 6 weeks after MMR vaccination and the attributable risk of ITP within 6 weeks after MMR vaccination.

Methods Using the General Practice Research Database we identified children with a first-time diagnosis of ITP from a base population of children aged less than 6 years between January 1988 and December 1999. After describing the characteristics of all the children identified with ITP, we focused on cases aged 13-24 months to perform a population-based, case-control analysis to estimate the relative risk of developing ITP within 6 weeks after MMR vaccination. We also calculated the risk of ITP attributable to the MMR vaccination.

Results Sixty-three children with a first time diagnosis of ITP were identified; 23 cases were between 13 and 24 months old. The relative risk estimate for ITP within 6 weeks after MMR vaccination, compared to the combined group of unvaccinated children and children vaccinated with MMR more than 26 weeks previously was 6.3 (95% CI 1.3-30.1). The attributable risk of developing ITP within 6 weeks after MMR vaccination was estimated to be 1 in 25 000 vaccinations (95% confidence interval 21 300, 89 400).

Conclusion This study confirms the increased risk of ITP within 6 weeks after MMR vaccination. However, the attributable risk of ITP within 6 weeks after MMR vaccination is low.